I had an aunt like your sister in law. Lovely kind person but boy could she talk. Forever and ever about shite this and that. Honestly that's very hard work for anyone to tolerate.
Eventually I realised that auntie would talk and talk but didn't care if you were listening or not, she would rattle on and on even when I closed my eyes and dozed during her monologues. I once left the room to get tea ready and I could hear her talking to me even though I wasn't there!
The only kind semi solution I could muster was to either fall asleep in her presence, or pretend to, or leave the room and get a bit of respite. It is exhausting trying to be social with someone like this, because they DON'T CARE about your response, or whether you are actually listening or not. I'm convinced that such people cannot judge the social signals at all, so the best thing is to mutter a few umms and aaahs, stick it out for a while then leave the room. And continue to do this over and over with longer spells out of the room altogether.
I left her magazines, and the TV zapper when I'd leave, but she often followed me out, not to help but to keep talking OMFG! I can laugh about it now as she died recently, but we all said that she must have Jesus's ears bent at this stage too.
Sympathies OP, sympathies.